The present invention relates to the use of tin derivatives as polycondensation catalysts. The use of tin salts as polycondensation catalysts has been known for several decades. See F. Hostettler and E. F. Cox (1960) Ind. Eng. Chem. 52:609. These derivatives are used in particular for the condensation of silicones and for the production of polyurethanes.
In the last few years demand has increased for coating systems capable of both extended viscosity stability, that is pot life, and high productivity. High productivity refers to the ability to produce the desired polymer, or polycondensate, as quickly as possible under the implementation conditions, for example, in automotive refinish applications.
For environmental reasons, governmental agencies and users complying with governmental regulations worldwide have exerted pressure to develop coating systems having lower levels of volatile organic compounds (VOC's). A key to resolving these issues is through the dramatic reduction or the elimination of solvents used in coatings. However, reducing the amount of solvent will negatively affect pot life, if cure times remain constant, unless a latent catalyst system is developed.
Jousseaume, B. et al., (“Air Activated Organotin Catalysts for Silicone Curing and Polyurethane Preparation” (1994) Organometallics 13:1034), and Bernard, J. M. et al. (U.S. Pat. No. 6,187,711) describe the use of distannanes as latent catalysts, e.g. Bu2(AcO)SnSn(OAc)Bu2. Upon exposure to air, such species oxidize to give distannoxanes, e.g. Bu2(AcO)SnOSn(OAc)Bu2, which are known to be highly active for urethane formation. However, the carboxylate-substituted distannanes are themselves catalysts for the reaction, and have been reported to be “relatively stable in air”, which suggests that oxidation to form an active catalyst is slow. See U.S. Pat. No. 3,083,217 to Sawyer et al. UV light appears to be necessary in order to induce oxidation at an appreciable rate in these distannanes. Thus, there exists a need for a catalyst precursor that, in the absence of air, is a very poor catalyst and yet, upon exposure to air, rapidly forms a highly active catalyst that allows for rapid cure.